Different genetically engineered mutants of bovine viral diarrhea virus (BVDV) were analyzed for the ability to establish infection in the fetuses of pregnant heifers. The virus mutants exhibited either a deletion of the overwhelming part of the genomic region coding for the N-terminal protease N pro , a deletion of codon 349, which abrogates the RNase activity of the structural glycoprotein E rns , or a combination of both mutations. Two months after infection of pregnant cattle with wild-type virus or either of the single mutants, the majority of the fetuses contained virus or were aborted or found dead in the uterus. In contrast, the double mutant was not recovered from fetal tissues after a similar challenge, and no dead fetuses were found. This result was verified with a nonrelated BVDV containing similar mutations. After intrauterine challenge with wild-type virus, mutated viruses, and cytopathogenic BVDV, all viruses could be detected in fetal tissue after 5, 7, and 14 days. Type 1 interferon (IFN) could be detected in fetal serum after challenge, except with wild-type noncytopathogenic BVDV. On days 7 and 14 after challenge, the largest quantities of IFN in fetal serum were induced by the N pro and RNase-negative double mutant virus. The longer duration of fetal infection with the double mutant resulted in abortion. Therefore, for the first time, we have demonstrated the essential role of both N pro and E rns RNase in blocking interferon induction and establishing persistent infection by a pestivirus in the natural host.

Pestiviruses represent important pathogens of farm animals that have evolved unique strategies and functions to stay within their host populations. E rns , a structural glycoprotein of pestiviruses, exhibits RNase activity and represents a virulence factor of the viruses. E rns forms disulfide linked homodimers that are found in virions and virus-infected cells. Mutation or deletion of cysteine 171, the residue engaged in intermolecular disulfide bond formation, results in loss of dimerization as tested in coprecipitation and native protein gel electrophoresis analyses. Nevertheless, stable virus mutants with changes affecting cysteine codon 171 could be recovered in tissue culture. These mutants grew almost as well as the parental viruses and exhibited an RNase-positive phenotype. E rns dimerization-negative mutants of classical swine fever virus were found to be attenuated in pigs even though the virus clearly replicated and induced a significant neutralizing antibody response in the animals.

The classical swine fever virus (CSFV) represents one of the most important pathogens of swine. The CSFV glycoprotein E is an essential structural protein and an important virulence factor. The latter is dependent on the RNase activity of this envelope protein and, most likely, its secretion from the infected cell. A further important feature with regard to its function as a virulence factor is the formation of disulfide-linked E homodimers that are found in virus-infected cells and virions. Mutant CSFV lacking cysteine (Cys) 171, the residue responsible for intermolecular disulfide bond formation, were found to be attenuated in pigs (Tews BA, Schürmann EM, Meyers G. J Virol 2009;83:4823-4834). In the course of an animal experiment with such a dimerization-negative CSFV mutant, viruses were reisolated from pigs that contained a mutation of serine (Ser) 209 to Cys. This mutation restored the ability to form disulphide-linked E homodimers. In transient expression studies E mutants carrying the S209C change were found to form homodimers with about wt efficiency. Also the secretion level of the mutated proteins was equivalent to that of wt E. Virus mutants containing the Cys171Ser/Ser209Cys configuration exhibited wt growth rates and increased virulence when compared with the Cys171Ser mutant. These results provide further support for the connection between CSFV virulence and E dimerization.